Steady-state Background Currents

As with the effects of oxidant stress on the calcium channel, part of the change in the steady-state background current could also be attributed to an indirect effect secondary to the elevation of intracellular calcium (Matsuura and Shattock, 1991b). However, oxidant stress also exerted a direct effect on the inward rectifying potassium current (7ki). The combination of an inhibition of 7ki and the activation of a calcium-dependent current are likely to contribute to the prolongation of the action potential duration and the increased susceptibility... 

Matsuura, H. and Shattock, M. J. (1991b). Effects of oxidant stress on steady-state background currents in isolated ventricular ceUs. Am. J. Physiol. 261, H1358-H1365. 

To ensure a linear response of the system, the amplitude of the perturbation voltage needs to be small, usually 5-10 mV for photoelectrochemical devices. Such a small perturbation also gives a very small current response. The first job of the FRA is to isolate these signals from the background signal. The latter consists of the steady-state (dc) current and voltage plus the unavoidable electrical noise. This seems an impossible task, since the signals that we are interested in can easily... 

ICA trimer films were produced by the electropolymerisation of 100 mM ICA in background electrolyte at + 1.46 V at the RDE, rotating at 4 Hz. Under these conditions [8,9], electrooxidation and film production occurs under steady-state current conditions at close to 100% current efficiency, and the film is expected to consist almost entirely of free trimer. The electrooxidation reaction was terminated after passing a charge of 173 mC. From equation (11.1) and (11.23) we calculate that this corresponds to 25 mC of redox charge or approximately 250 nmoles of deposited trimer. 

It is interesting to estimate the effective tip radius immersed in the water layer, which is responsible for a tip current of 1 pA at 1.5 V bias. As shown in Fig. 11, a polyurethane-coated W tip behaves as a microelectrode. A sigmoidal diffusion-limited current superimposed on the linear background current was obtained for the reduction of 1 mM Ru(NH3)g+ in 10 mM NaC104 solution. An effective radius estimated from the nearly steady-state current is 3 /xm. Also shown in Fig. 11 is the anodic background current due to the oxidation of W at potentials positive of 0.4 V versus SCE (curve b). From the data shown in curve c of Fig. 10 and curve b of Fig. 11, if one assumes that similar effective tip radius is responsible for both anodic and cathodic redox processes, an estimated effective contact radius of 3 nm can be obtained for a background current flow of 1 pA at a bias voltage of 1.5 V. 

Of the three SECM modes that can be used to study electrode reaction mechanisms—the TG/SC, feedback, and SG/TC modes—the former is the most powerful for measuring rapid kinetics. With this approach, fast followup and sandwiched chemical reactions can be characterized under steady-state conditions, which are difficult to study even with rapid transient techniques such as fast scan cyclic voltammetry or double potential step chronoamperometry, where extensive corrections for background currents are often mandatory (44). At present, first- and second-order rate constants up to 105 s 1 and 1010 M 1 s, respectively, should be measurable with SECM. The development of smaller tip and substrate electrodes that can be placed closer together should facilitate the detection and characterization of electrogenerated species with submirosecond lifetimes. In this context, the introduction of a fabrication procedure for spherical UMEs with diameters... 

Actually one need not even apply steps. It is satisfactory to change the potential linearly with time and to record the current continuously, as long as the rate of change is small compared to the rate of adjustment in the steady state. Section 6.2.3 contains a discussion of the required conditions in more quantitative terms. Virtually all sampled current voltammetry at UMEs is carried out experimentally in this linear-sweep form, but the results are the same as if a normal sampled-current voltammetric protocol were employed, except with respect to the charging-current background [see Sections 6.2.4 and 7.3.2(c)]. 

Note that in this case the current does not decay to zero (or the background level), as in the case of the unperturbed reaction (k 0), but instead decays to a steady-state value, where... 

Figure 13 shows the steady-state A-E curves for the redox reaction of Cyt c at a polypyrrole-methylene blue CME." From Fig. 13, the redox processes of Cyt c, which cannot be detected by cyclic voltammetry owing to the large background current, can be directly monitored. The values of A p obtained from the inflection points is about 60 mV. Because A p < 200 mV, Eq. (25) is used to determine itjj, which gives a value of 5.4 X 10 cm sec . " The determination of kinetic parameters for other redox proteins by CPS/SSCA at various types of CMEs is discussed in the following section. 

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